A conventional broadband antenna, that produces Gaussian-like beams having a single polarization direction, is described in U.S. Pat. No. 7,889,148. The conventional broadband antenna includes a ridge horn and a waveguide which is at least partially inside the ridge horn. The waveguide includes a central dielectric slab, an upper slab above the central dielectric slab and a lower slab below the central dielectric slab. These slabs have a lower dielectric constant compared with the central one. The central slab increases in width toward the ridge horn and its thickness is substantially constant and less than a quarter of a wavelength at the highest frequency of operation. The conventional broadband antenna can be used in a measurement system for providing a substantially plane wave interaction between the electromagnetic wave and a sample at an operation frequency ranging from 0.7 GHz to 20 GHz.